In recent years, the development of chemical reaction processes utilizing forest resources and biomass instead of petroleum resources has thrived and sugar alcohols derived from cellulose and the development of their applied uses have attracted attention. One example of this is a process for dehydrating a sugar alcohol to obtain the raw material of a useful chemical substance. For example, an isosorbide that is a dianhydrosugar alcohol obtained by dehydrating sorbitol obtained by cellulose decomposition is useful for the production of raw materials used in medicinal compounds and the production of condensation polymers such as polyurethanes, polycarbonates and polyesters, and therefore catalysts for improving its reaction yields have been actively developed as shown, for example, in Patent Literature 1.
The reaction for obtaining isosorbide from sorbitol comprises a two-stage dehydration reaction as shown by the chemical reaction formula in FIG. 1. Sorbitol has six hydroxyl groups in the molecule, and therefore many types of intermediates are produced by the dehydration reaction of sorbitol, and particular ones among them can produce isosorbide. There are many types of side reactions in this manner, and therefore there are great difficulties involved in the development of a catalyst for increasing the yield of isosorbide, and a chemical reaction control technique using said catalyst.
For example, in Patent Literature 1, regarding the reaction for obtaining isosorbide from sorbitol, many experiments are performed using, as catalyst for the dehydration of sorbitol, a sulfuric acid that is a homogeneous acid catalyst or zeolites as heterogeneous catalysts. Among said experiments, for various zeolites, a maximum yield of only about 38% is obtained in a reaction time as long as 12 hours (Example 13), whereas for sulfuric acid, a yield of more than 70% is obtained in a reaction time of 75 minutes (Example 3). In this manner, sulfuric acid gives a very high yield compared with zeolite catalysts in a shorter amount of time, and therefore it has been widely used as a catalyst for isosorbide production.
Despite the high yield and low cost, using a sulfuric acid catalyst in this manner poses many industrial problems. In particular, there are a number of factors that can increase costs. For example, because it is a homogeneous catalyst, it is necessary to separate and purify the isosorbide after completion of the reaction, and the cost of the equipment and steps needed for said separation and purification is considerable. In addition, the aforementioned separation and purification further entail the complicated issue of the neutralization and disposal of the separated and remaining waste sulfate. Furthermore, appropriate steps must be taken to prevent the corrosion of the chemical reaction apparatus by sulfuric acid.
Therefore, the development of techniques for minimising the drawbacks of sulfuric acid has also been promoted, and as an example thereof, an attempt to use various solid (heterogeneous) catalysts is shown in the above Patent Literature 1. In Patent Literature 1, for various zeolites such as H-β zeolites (acid type β zeolites), H-mordenites and H-ZSM-5, as solid acid catalysts, the isosorbide production yield is measured while modifying conditions such as the Si/Al ratio, reaction time and the amount of the acid catalyst added. The excerpted and summarized results are shown in the following Table 1. Among these solid catalysts, the use of the H-type β zeolite with Si/Al=12.5 produces the highest yield (38%) in a reaction time of 12.3 hours. The zeolite-based catalysts in Table 1 need a very long reaction time and have very low yield, compared with the sulfuric acid catalysts used which yield more than 70% in a reaction time of 75 minutes, and therefore the zeolite-based catalysts have poor practicality. In Patent Literature 1, for sulfated zirconia, a high yield of 73% is obtained (Example 23), but the durability is low, and therefore sulfated zirconia is not practical for mass synthesis uses.
TABLE 1Excerpts of Data in Examples and Comparative Examples inPatent Literature 1Amount of Total yieldReactioncatalystReactionIsosorbideof by-timeadded (%temperatureExampleNameSi/Alyieldproducts(hours)by weight)(° C.)Example 11H-β12.518.026.45.09.3150.0Example 12DAY-5555.010.030.05.19.4150.0Example 15H-Y25.01.321.15.19.4150.0ComparativeUSY2.80.00.45.09.3150.0Example 18Example 17H-ZSM-515.00.21.55.09.4150.0ComparativeH-ZSM-525.00.08.95.09.3150.0Example 20ComparativeH-ZSM-575.00.03.25.09.5150.0Example 21Example 16H-mordenite45.00.822.95.19.3150.0ComparativeH-mordenite15.00.01.45.09.3150.0Example 22Example 13H-β12.538.01.912.320.0150.0
Considering that the dominant factor of the dehydration reaction from sorbitol to isosorbide is the high acidity resulting from the sulfuric acid catalyst, it is expected that, the yield of isosorbide will also increase for these various zeolites, when those having a high acidity are used as catalysts. Their acidity depends on both the concentration of active sites and the intensity of each activity. It is considered that the acidity of a zeolite as a Bronsted acid depends on the entry of Al into the Si network, and it is said that there is a large acid content in a region in which the Al composition is relatively large, i.e. a region in which the Si/Al atomic ratio is small. For example, in Patent Literature 2, zeolites are studied as solid acid catalysts for the dehydration reaction of an alcohol, and in its specification, it is disclosed that the acidity is highest in the range of Si/Al=5 to 20 (paragraph [0009], Patent Literature 2).
From the above point of view, for the data in the Examples and the Comparative Examples in Patent Literature 1 (Table 1), the isosorbide yield η1 and the total yield of by-products η2 with respect to the Si/Al ratio are plotted as shown in FIG. 2. Here, no fixed relationship is noted between the yield of isosorbide η1 and the Si/Al ratio, and a tendency for one with a high isosorbide yield η1, to also have a high yield of by-products η2 is noted.
In addition, for the H-type β zeolites, a relatively high yield (38%) is obtained at Si/Al=12.5 as described above, but an impractically long reaction time of 12 hours is required, about 10 times longer than that required for the sulfuric acid catalyst. From the above data based on Patent Literature 1, it seems that it is difficult to obtain, with a solid catalyst having low price and good durability, a yield of about 70% at 150° C. or less in a reaction time of about 1 to 3 hours as with the sulfuric acid catalyst.
With respect to the durability of the solid catalyst described above, both durability for high temperature, chemical action and the like during the reaction, and durability for regeneration treatment performed when the activity decreases are important. In a dehydration reaction with a solid acid catalyst, generally, with the progress of the reaction, the reaction active sites are covered due to carbon adhesion (caulking), causing a decrease in yield. In order to reduce activity deterioration due to caulking, it is important to decrease the dehydration reaction temperature as much as possible. But, when caulking occurs nevertheless, recycling in which the catalyst is subjected to washing and carbon removal is performed to extend the life of the catalyst. Therefore, it is very important in promoting practical use that the deterioration of the solid catalyst is minimised in the recycling step, and the repeated regeneration is stable.
As described above, the need to replace the conventional sulfuric acid catalyst by a solid acid catalyst having excellent handleability is high, but the fact is that a catalyst satisfying performance requirements such as a high reaction yield, recyclability and cost cannot be implemented yet.